Message In A Cake?

So I have had a request to make a cake for parents to ask some friends of theirs to be godparents. She wants somehow to do this with cake in a cute way other than just writing it on the top... I was wondering has anyone ever done a message IN a cake somehow? I was thinking it would be cute for the receivers to cut in to the cake and find a little surprise! How would I do this?

if you could figure a way to put it IN the cake you would then have to figue a way for it not to be cut in the wrong spot. Maybe you could put a present topper on it or something and have them be able to open the present topper to find a note inside? Or place a ribbon around the cake and when they remove the ribbon to cut into the cake it could just be written under the ribbon?